Photorefractive keratectomy is a procedure in which excimer lasers are used to correct optical errors of the eye, such as myopia, near-sightedness, hyperopia, far-sightedness and astigmatism. One typical procedure is to remove corneal tissue using a laser configured at 193 nanometers, although other wavelengths may also be used. Each pulse of the laser removes a small amount of corneal tissue and, by controlling the number of pulses and exposure pattern of the laser, the cornea can be reshaped as desired. For example, to correct near-sightedness, more tissue is removed from the center than at the edge, so that there is an overall flattening of the cornea.
An initial step in the procedure is to remove from the cornea the surface layer of cells known as the epithelium. The epithelium, typically about fifty microns thick, covers and protects the underlying tissue, principally collagen, that makes up the bulk of the cornea. In the past, the epithelium has been removed by scraping with a mechanical device, such as the edge of a blade or other surgical instrument. This has a number of disadvantages.
For example, the use of any mechanical instrument presents some risk of infection, and mechanical removal is also inherently irregular and highly dependent on the skill of the person accomplishing the procedure. Scraping may injure the underlying cornea, e.g., by causing nicks or scratches which may in turn affect the smoothness of the later removal of the underlying collagen, and small "is-lands" of epithelium may remain after it is thought that all the epithelium has been removed or, in the course of scraping away the epithelium, some of the underlying collagen may be removed also. Moreover, to assure that the entire area to be treated has been exposed, it is usually necessary to remove the epithelium from an area that is larger than that to be treated. This is undesirable since, among other things, a larger area requires longer to heal and results in an increased risk of infection. For example, if the area of the collagen 16 to be treated is 5 millimeters in diameter, using mechanical ablation techniques it is usually necessary to remove the overlying epithelium 14 from a 6 or even a 7 millimeter zone. This result is that the overall exposed area is between about one and a half and two times the size of the treatment sone. Further, if the epithelium is mechanically removed, the laser used to ablate the underlying collagen cannot be positioned until after the epithelium removal has been completed, and this undesirably increases potential dehydration and the overall length of the surgical procedure.